Pseudoexcitation

Keywords Cycloadditions, Chemical orbital theory. Donor-acceptor interaction. Electron delocalization band. Electron transfer band, Erontier orbital. Mechanistic spectrum, NAD(P)H reactions. Orbital amplitude. Orbital interaction. Orbital phase. Pseudoexcitation band. Quasi-intermediate, Reactivity, Selectivity, Singlet oxygen. Surface reactions... 

The pseudoexcitation in donors occurs through the d-a -d interaction. The a-d-a orbital interaction causes the pseudoexcitation in the acceptors. The simultaneous pseudoexcitations in the donors and acceptors are caused by the a-d-a -d interaction (Scheme 2). 

Much stronger donor-acceptor interactions stabUze D+A too much to give rise to the pseudoexcitation. The electron transferred configuration is stable and predominant. Electrons transfer to generate ion radical pairs or salts. Covalent bonds do not form and electron transfer results. 

Delocalization band Pseudoexcitation band Transfer band... 

With the power of the donors and acceptors, changes occur in the important frontier orbital interactions (Scheme 2) and in the mechanism of chemical reactions. The continuous change forms a mechanistic spectrum composed of the delocalization band to pseudoexcitation band to the electron transfer band. 

Scheme 4 Thermal [2+2] cycloaddition reactions in the pseudoexcitation band...

The [2h-2] cycloaddition could occur thermally in the pseudoexcitation band. In fact, an alkyne with electron-donating group, ethoxyacetylene, and electron accepting carbonyl compound, perfluoroacetone, form the oxetene at low temperature (-78 °C) without light irradiation (pseudoexcitation band in Scheme 6) [26, 27],... 

Thermal [2h-2] cycloaddition reactions of carbonyl compounds were catalyzed by a Lewis acid. The catalyst forms complexes with the carbonyl compounds and enhances the electron-accepting power. The reaction shifts from the delocalization band to the pseudoexcitation band. Catalyzed [2h-2] cycloaddition reactions were observed with acetylenic compounds [28] and ketenes [29-31]. 

Olefins (enamines) unsymmetrically substituted with strong electron-donating (amino) group and CS generate zwitterions (1,4-dipoles) [32, 33]. Polar additions are proposed here to be reactions in the pseudoexcitation band. 

A stronger donor, the butadiene with the amino groups in place of the methoxy group in the 1,4-positions, was calculated to react with TCNE via a zwitterion (pseudoexcitation band in Scheme 7) [34], The loss of the stereochemical integrity was observed inthe[4+2]cycloadditionreactionsbetweensomestrongdonors,l,4-bis(dimethylamino) butadienes, and acceptors, fumaric and maleic dinitriles [36],... 

According to the calculations at high levels of theory, the [4+2] cycloaddition reactions of dienes with the singlet ( A oxygen follow stepwise pathways [37, 38], These results, which were unexpected from the Woodward-Hoffmann rule and the frontier orbital theory, suggest that the [4+2] cycloadditions of the singlet ( A oxygen could be the reactions in the pseudoexcitation band. 

A Mechanistic Spectram of Chemical Reactions 1.4.2 Pseudoexcitation Band... 

Trauner and colleagues [39] recently found a striking contrast in the thermal and catalyzed reactions of a triene. Thermal reaction of a trienolate readily underwent disrotatory electrocyclization to afford cyclohexadiene (delocalization band in Scheme 8) in accordance with the Woodward-Hoffmann rule. Surprisingly, treatment of the trienolate with Lewis acid did not result in the formation of the cyclohexadiene but rather gave bicyclo[3.1.0]hexene in a [4n +2nJ manner (pseudoexcitation band in Scheme 8). The catalyzed reaction is similar to the photochemical reaction in the delocalization band. 

The hexatriene is polarized by unsymmetrical substitution with the C=0 group, and further activated by coordination with Lewis acid. The catalyzed reaction is polar. The similarity between the catalyzed and the photochemical reactions can be understood if polar reactions belong to the pseudoexcitation band as has been proposed in Sect 1. 

Electrophilic aromatic substitution reactions take place between aromatic compounds and strong acceptors (pseudoexcitation band in Scheme 9). The substitutions are... 

The theory of the mechanistic spectrum generally snggests that photochemical reactions between donors and acceptors in the delocalization band could be similar to thermal reactions between strong donors and acceptors in the pseudoexcitation band. This is fnrther snpported by the reactions of indoles with electron-accepting... 

A stronger acceptor, TCNE, nndergoes a similar reaction without irradiation to give tricyanovinylindole after the elimination of HCN by pyridine (pseudoexcitation band in Scheme 10) [47]. 

Some typical reactions in the pseudoexcitation band are reviewed in this section. The importance of pseudoexcitation [1] in chemical reactions was supported by the detailed numerical analysis of the electronic structures of the transion states [66]. The concept of pseudoexcitation appeared in physics [67-69]. 

Singlet molecular oxygen ( A is an electron acceptor powerful enough to react with olefins in the pseudoexcitation band. The [2h-2] cycloaddition and ene reactions and the stereoselectivities are reviewed in this subsection. 

Cycloaddition reactions can occur with retention of configuration in the pseudoexcitation band (Sect 1.1) whereas [2jt H-2jtJ reactions are symmetry-forbidden in the delocalization band. Experimental evidence is available for the stereospecific [2-1-2] cycloaddition reactions between A and olefins with retention of configuration (Scheme 14) [82]. A perepoxide intermediate was reported to be trapped in the epoxide form [83] in the reaction of adamantylideneadamantane with singlet oxygen affording dioxetane derivatives [84]. 

It is noteworthy that these are the reactions in the pseudoexcitation band if the polar reactions are taken as proposed in Sect I. 

Bending of unsaturated bonds reduces the overlap between the p-orbitals and weakens the interaction. The n orbital hes high in energy and the Jt orbital lies low. Bent unsaturated bonds are electron acceptors as well as donors. The energy gap between Jt and Jt is small. Bent unsaturated bonds are readily pseudoexcited to undergo [2+2] cycloaddition reactions. 

Ketenes have cnmnlative bonds and can undergo [2+2] cycloaddition reactions across C=C and C=0 bonds. Interestingly, most of the prodncts obtained are cyclobutanones rather than oxetanes. Thermal [2+2] cycloaddition reactions in the pseudoexcitation band occur between electron donors and acceptors. Alkenes are donors while ketenes are acceptors. In contrast to the experimental observations. 

Scheme 25 Pseudoexcitation in the [2+2] cycloaddition reactions of ketenes with alkenes...

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